System for high efficiency solid-state light emissions and method of manufacture

ABSTRACT

In one embodiment of the invention, a bonding material is used to bond a substitute substrate to the LED, wherein the bonding material does not including gold or tin. The bonding material preferably includes gallium (Ga), such as a combination of Ga and Al or Cu. This bonding material has high thermal conductivity, high strength, high temperature stability and is low cost. In another embodiment of the invention, the substitute substrate is first thinned before it is bonded to the LED structure, so that the substitute substrate is flexible and conforms to the shape of the LED structure. In yet another embodiment of the invention, an apparatus is used for bonding a substitute substrate to a LED which includes a plurality of semiconductor epitaxial layers, said semiconductor epitaxial layers having been grown on the growth substrate so that said semiconductor epitaxial layers are curved in shape. The apparatus includes a conduit for evacuating a region near the substitute substrate on a side of the substitute substrate that is opposite to that of said semiconductor epitaxial layers. Gas pressure is applied on the semiconductor epitaxial layers, and the substitute substrate conforms to the shape of said semiconductor epitaxial layers as a result of pressure applied. A bonding material is used for bonding said substitute substrate to the semiconductor epitaxial layers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the application entitled: “HIGH LIGHTEFFICIENCY SOLID-STATE LIGHT EMITTING STRUCTURE AND METHODS TOMANUFACTURING THE SAME,” by Lee et al., application Ser. No.: U.S. Ser.No. 11/777,987 filed on Jul. 13, 2007, which claims the benefit of thebenefit of U.S. provisional patent application No. 60/937,245 entitled“High Light Efficiency Solid-State Light Emitting Structure And MethodsTo Manufacturing The Same”, filed on Jun. 25, 2007. Both of the aboveapplications are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

Over the last decade, the advent of solid-state lighting has led torapid advances in the production of high brightness Light Emitting Diode(LED). LED's brightness is now competing with incandescent andfluorescent light sources mainly due to breakthroughs on chipstructures, improved extraction and thermal management of LED lightingsystem.

The light extraction efficiency reflects the ability of photons emittedinside the LED chip to escape into the surrounding medium. For example,the index of refraction of Gallium phosphide-based materials is close to3.4, compared with 1 for air and 1.5 for epoxy. This results in acritical angle of 17° for air and 25° in epoxy, respectively. If asingle interface is considered only 2% of the incident light into airand 4% into epoxy will be extracted. As a comparison, the index ofrefraction of Gallium nitride-based materials is close to 2.3. Thisresults in a critical angle of 26° into air and 41° into epoxy. If asingle interface is considered only 5% of the incident light into airand 12% into epoxy will be extracted. The rest is reflected into thesemiconductor where it will eventually be reabsorbed or recycled andresults in the performance degradation of the device.

While light extraction efficiency is an important consideration in thedesign of LEDs, other factors may also be important. For example, toensure that the entire active layer in the LED is utilized in lightemission, it is desirable to spread the electrical current to the entireactive layer. To enhance the efficient use of electrical current inlight generation, the ohmic contact resistance with the LED should alsobe as low as possible. To enhance light extraction, the layers betweenthe active layer and the emitting surface of the LED should have highlight transmission characteristics. In addition, in order to efficientlyreflect light generated by the active layer traveling in directions awayfrom the light emitting surface of the LED, the different layers of thelight reflector employed should have high index contrast.

One type of reflectors for LEDs is proposed in the paper“Omni-Directional Reflectors for Light-Emitting Diodes,” by Jong QyuKim, et al. Proc. of SPIE Volume 6134, pages D-1 to D-12, 2006. In FIG.5 of this paper, a GaInN LED with an omni-directional reflector (ODR) isshown. This LED structure comprises a sapphire substrate supporting aGaInN LED. A thin layer of oxidized Ruthenium (Ru) is used as asemi-transparent low-resistance p-type ohmic contact. A quarter-wavethick silicon dioxide low-refractive index layer perforated by an arrayof silver micro-contacts and a thick silver layer are also employed. Insection 3.3.3 on page D-9 of this paper, however, the authors Kim, etal. indicated that the above structure of FIG. 5 is disadvantageousbecause the above design “needs absorptive semi-transparent currentspreading layer, such as RuO₂, . . . , which leads to a decrease inreflectivity of the ODR.” Furthermore, the refractive index of silicondioxide is deemed to be not low enough for high refractive indexcontrast with high-index semiconductor materials, which limits furtherimprovement of light extraction efficiency in GaN-based LEDs.

As an alternative, the authors proposed an ODR structure illustrated inFIGS. 11 and 12 of the paper. In this alternative ODR structure, theoxidized Ruthenium and silicon dioxide layers in FIG. 5 are replaced byan indium-tin oxide (ITO) nanorod low index layer illustrated in FIG. 12of the paper. However, as illustrated in FIG. 13 of the paper, the ITOnanorod layer provides mediocre ohmic contact characteristics. Moreover,the ITO material reacts strongly with metal, such as silver. When theITO nanorod layer proposed by Kim, et al. comes into contact with asilver substrate underneath, interdiffusion occurs at the interfacewhich greatly reduces the reflective properties of the resultingstructure. This will also greatly reduce the light extraction efficiencyof the LED. It is therefore desirable to provide an improved LEDstructure in which the above-described difficulties are alleviated.

Thermal management has always been a key aspect of the proper use ofLEDs. Poor thermal management leads to performance degradation andreduced lifetime of LEDs.

A substrate of high thermal conductivity becomes a necessity for theoperation of high power LEDs. It allows heat generated at the chip levelto be transferred efficiently away from the chip through the substrate.Given that conventional red (AlGaInP) and blue (InGaN) LED is grown fromN+ GaAs and sapphire substrates, respectively, one of the majordrawbacks of GaAs and sapphire are their poor thermal conductivity. GaAsand sapphire have thermal conductivity values of 50, and 40 w/m K,respectively. Obviously, replacing GaAs or sapphire with a carrier ofhigh thermal conductivity such as one made of Si (150 W/m□K), Cu (400W/m□K), or AlSiC (180 W/mK) can significantly improve the LEDperformance through better heat dissipation.

The substrate from which the LED is grown is referred to herein as thegrowth substrate. The high thermal conductive substrate from which theLED is transferred to is referred to herein as the substitute substrate.

To create good bonding with growth substrate, it is necessary to have asubstitute substrate with CTE closely match with that of growthsubstrate. In the case of GaN based LED grown from sapphire substrate,the fabrication process introduces considerable compressive stress onthe LED due to the slightly higher CTE of sapphire than GaN. When thesapphire substrates are replaced with substitute substrate, the LED maybe damaged if such compressive force is released rapidly. It is wellknown that GaN based LED materials are strong under compression and weakunder tensile force. Therefore, it is desirable to preserve thecompressive force on the LED to enhance the reliability of LED chips forsubsequent thermal processes such as laser lift-off and die bonding. Asa result, it is desirable to use a substrate material that has CTE equalto or slightly greater than sapphire substrate (˜6 ppm/K) to replace thesapphire growth substrate.

Metal matrix composites are well known material that typically includesa discontinuous particulate reinforcement phase within a continuousmetal phase. An example is silicon carbide reinforced aluminum matrixcomposite, AlSiC, which is made by infiltrating porous silicon carbidewith molten aluminum. The AlSiC metal matrix composite system has thepositive attributes of high thermal conductivity, low and tailorablecoefficient of thermal expansion and high strength. These attributesrender AlSiC suitable as a substitute substrate.

Bonding material is another important part of the LED system, since thebonding phase will not only play an important part to the thermalmanagement system but also need to survive the subsequent chipprocessing processes such as laser lift-off, and die bonding processes.Ideally, the bonding material has high thermal conductivity, highstrength, high temperature stability and is low cost. In the case ofGaN/sapphire system, the LED growth wafer usually is not smooth, partlydue to the inherent stress between sapphire and GaN and partly due tothe particulate contamination in the growth process. To fully cover thenon-smooth surface of growth wafer, solder bonding method is thepreferred method. Among various solder bonding materials, Sn based softsolders are relative cheap but have low strength and low temperaturestability; hard solders such as AuSn or AuGe have high strength, hightemperature stability but are high cost and have relatively low thermalconductivity. It is therefore desirable to develop a better bondingmaterial that does not have the above shortcomings.

To bond a substitute substrate to a growth substrate, it is necessary toprovide good contact between the bonding wafers. In the case ofGaN-based LEDs grown from sapphire substrates, the GaN growth processintroduces considerable stress on the LED due to the CTE mismatchbetween sapphire and GaN material. As shown on FIG. 5, the stresscreated ˜30 um curvature from center to the edge of the GaN/sapphiregrowth wafer. Obviously, it is difficult to generate good bondingbetween a flat substitute wafer and a curved growth wafer. Poor bondingbetween growth substrate and substitute substrate will not only resultin lower yield during the sapphire removal process but also createreliability problem on LED chips. Commercial wafer bonding devices andtechniques are designed to handle flat wafers, not wafers with curvedsurfaces. It is therefore desirable to develop a better wafer-bondingdevice/process to overcome the unique bonding problem.

SUMMARY OF THE INVENTION

As noted above, tin (Sn) based soft solders are relative cheap but havelow strength and low temperature stability, while hard solders such asgold-tin (AuSn) or gold-germanium (AuGe) have high strength, hightemperature stability but are high cost and have relatively low thermalconductivity. Thus, in one embodiment of the invention, a bondingmaterial is used to bond a substitute substrate to the LED, wherein thebonding material does not including gold or tin. The bonding materialpreferably includes gallium (Ga), such as a combination of Ga and Al orCu. This bonding material has high thermal conductivity, high strength,high temperature stability and is low cost.

To improve the bonding between the flat substitute substrate and an LEDthat has a curved shape, in another embodiment of the invention, thesubstitute substrate is first thinned before it is bonded to the LEDstructure, so that the substitute substrate is flexible and conforms tothe shape of the LED structure. In one implementation of thisembodiment, the thinned substitute substrate is bonded to the LEDstructure by applying pressure between the substitute substrate and theLED structure and heat is applied to the bonding material between them.

In yet another embodiment of the invention, an apparatus is used forbonding a substitute substrate to a LED which comprises a plurality ofsemiconductor epitaxial layers, said semiconductor epitaxial layershaving been grown on the growth substrate so that said semiconductorepitaxial layers are curved in shape. The apparatus comprises a conduitfor evacuating a region near the substitute substrate on a side of thesubstitute substrate that is opposite to that of said semiconductorepitaxial layers. Gas pressure is applied on the semiconductor epitaxiallayers, and the substitute substrate conforms to the shape of saidsemiconductor epitaxial layers as a result of pressure applied. Abonding material is used for bonding said substitute substrate to thesemiconductor epitaxial layers.

The above features may be used individually or in combination.

All patents, patent applications, articles, books, specifications,standards, other publications, documents and things referenced hereinare hereby incorporated herein by this reference in their entirety forall purposes. To the extent of any inconsistency or conflict in thedefinition or use of a term between any of the incorporatedpublications, documents or things and the text of the present document,the definition or use of the term in the present document shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a cross sectional view of an LED with reflector layers of asolid state light emitting structure to illustrate an embodiment of theinvention.

FIGS. 1 b-1 g are views illustrating possible distributions of the viasin the solid state light emitting structure of FIG. 1 a.

FIG. 1 h is an exploded view of a portion of the distribution in FIG. 1e.

FIG. 2 is a cross sectional view of multiple LED chips with reflectorlayers on a substitute substrate to illustrate an embodiment of theinvention.

FIG. 3 is a graphical plot of the I-V characteristics of the compositemirror of FIGS. 1 a, and 2.

FIG. 4 is a graphical plot of the reflectivity of the composite mirrorof FIGS. 1 a, and 2.

FIG. 5 is a graphical plot of the curvature of GaN grown on sapphiresubstrate useful for illustrating an embodiment of the invention.

FIG. 6 a is an exploded view of some of an apparatus for bonding asubstitute substrate to a growth substrate with LED chips thereon.

FIG. 6 b is a schematic view of the apparatus of FIG. 6 a in use forbonding a substitute substrate to a growth substrate.

FIGS. 7 a and 7 b are graphical plots showing binary phase diagrams ofAl—Ga and Cu—Ga that illustrate characteristics of new bonding materialsfor illustrating an embodiment of the invention.

FIGS. 8 a and 8 b are SEM photographs of a plurality of LEDs before(FIG. 8 a) and after (FIG. 8 b) singulation, where the LEDs have beenprocessed according to an embodiment of the current invention.

FIG. 9 is a flow chart illustrating a process for making LEDs useful forillustrating an embodiment of the invention.

Identical components in this application are labeled by the samenumerals.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In some of the embodiments of the present invention, a light emittingstructure is provided with high light extraction efficiency, as well ashigh thermal dissipation properties. The structure and manufacturingprocesses provide three major advantages when compared to existingstructures and manufacturing methods:

1. A Composite Reflective Mirror is used to enhance the light extractionefficiency and total brightness output of the light-emitting device.

2. An innovative wafer bonding device/process is used to bond the curvedgrowth substrate with a flat substitute substrate. The process involvesusing a thin and flexible substitute substrate to conform to the curvedgrowth substrate and using isotropic gas pressure to generate uniformpressure across the wafer surface to create a good bonding betweensubstitute substrate and growth substrate. The light-emitting materialis supported by a new substitute substrate that has a CTE equal orslightly higher than the active epi-layers of the device (i.e., thelayers of the device that are grown using an epitaxial growth process).The substrate that supports the epi-layers is designed to provide robustmechanical support to the epi-layers as well as high thermalconductivity. Preferably the thermal conductivity of material in thesubstitute substrate is greater than about 100 W/m degrees Centigrade.

3. A cost effective bonding material that provides bonding betweengrowth substrate and substitute substrates has good thermal andmechanical properties.

Several different factors important in LED fabrication can all be takeninto account in one embodiment of a solid-state light emittingstructure. In this embodiment, a LED comprising a plurality ofsemiconductor epitaxial layers is supported by a substitute substratedifferent from the growth substrate. A current spreading layer betweenthe substitute substrate and the LED and in contact with said LED servesas an ohmic contact with the LED. A buffer layer between the currentspreading layer and the substitute substrate comprises a plurality ofvias and has a refractive index that is below that of the currentspreading layer. A reflective metal layer between the buffer layer andthe substitute substrate is connected to the current spreading layerthrough the vias in the buffer layer.

In the above embodiment, the buffer layer separates the currentspreading layer from the reflective metal layer. In the event that thematerial of reflective metal layer reacts with that of the currentspreading layer, the presence of a buffer layer reduces or prevents suchreaction. This is the case, for example, where the current spreadinglayer comprises ITO. Moreover, since the buffer layer has a refractiveindex that is below that of the current spreading layer, this increasesthe index contrast of the reflective mirror formed by the buffer layerand the reflective metal layer, and enhances the reflectivity and thusthe extraction efficiency of the LED device.

In another embodiment, a solid-state light emitting structure comprisinga plurality of semiconductor epitaxial layers grown on a growthsubstrate may be constructed as follows. A current spreading layer incontact with the LED is formed, where the current spreading layer servesas ohmic contact with the LED. A buffer layer is formed on the currentspreading layer, wherein the buffer layer comprises a plurality of holestherein. The buffer layer has a refractive index that is below that ofthe current spreading layer. A reflective metal layer is formed on thebuffer layer and the plurality of holes in the buffer layer are filledwith an electrically conductive material to provide vias, so that thereflective metal layer is electrically connected to the currentspreading layer through the vias. A substitute substrate in electricalcontact with the reflective metal layer is provided to support said LEDand the current spreading, buffer and reflective metal layers.

Composite Mirror:

The light emitting structure described in the present invention is asurface-emitting, solid-state light emitting structure, comprising a pnjunction light-generating region 100 (referred to as the active area) inFIG. 1 a, which region may include a p-doped layer and an n-doped layerwith an interface at 100 a. Region 100 is grown on a growth substrate 10comprising a suitable material such as sapphire or GaAs.

Light generated inside the active layers of a light emitting structuresuch as a light emitting diode is typically isotropically emitted withinthe epi-layers, meaning that photons are emitted in all directions, moreor less equally. The portion of the emitted photons that is emitted inthe upper hemisphere of the die is typically efficiently extracted fromthe device when proper surface texturing 129 is applied to the dies asillustrated in FIG. 2. However, the photons emitted in the lowerhemisphere of the device typically reach the bottom interface of thedevice, and they need to be redirected toward the upper surface of thedevice at the surface texturing 129 to optimize the light extractionefficiency of the surface emitting device. FIG. 2 also shows the n sidemetal ohmic contact 119.

As illustrated in FIG. 1 a, the reflecting structure of the presentinvention 105 includes the following features:

-   -   A contact layer 101 is preferably deposited on the p-doped        region of the p and n epi-layers 100 to form an ohmic contact.        This p-doped contact layer 101 may be formed by the deposition        of a stoichiometric Indium Tin Oxide layer (ITO). ITO is a good        candidate as the p-contact layer, due to its ability to form an        ohmic contact without annealing, its good electrical        conductivity, optical transparency, and its relatively high        refractive index (1.9). The following materials or combination        of the following materials also exhibit the desired properties:        Ni/Au and RuO₂ for example.    -   Following deposition of the ITO p-contact layer 101, one or more        dielectric layers 102 may be deposited in a sequence on the ITO        layer. Localized openings are formed by means of        photolithography used in semiconductor manufacturing through the        insulating dielectric layer or layers 102 to form an electrical        connection between the ITO layer 101 and the metal reflective        layer 103.    -   The reflecting structure is finally capped with a reflective        metal layer 103, which may be formed by a deposition process,        which not only deposits the metal layer 103 onto the dielectric        layer or layers 102, but also fills the openings or holes        therein to form vias 104 shown in FIGS. 1 a-1 h.

In prior devices, the ITO layer acts as a current-spreading layer forspreading current over the entire active region of the LED. In contrast,as illustrated in FIG. 1 a, the ITO 101 provides current spreadingbetween two adjacent holes only, not on the entire surface of thedevice. The lattice structure of the openings, the size of the openingsand their diameter are preferably optimized for maximum light extractionefficiency. FIGS. 1 b-1 g illustrate different possible configurationsof the openings or vias 104 networks. FIG. 1 h is an exploded view of aportion of the distribution in FIG. 1 e.

In one embodiment of the present invention, the thickness of ITO rangesfrom 10 to 500 nm, the diameter of the openings or vias 104 ranges from2 to 20 um and the spacing between vias ranges from 5 um to 100 um.

To obtain an optimal reflectivity of the structure, one importantsurface is the interface with the metallic layer 103. ITO depositionusually induces a rough surface morphology. Therefore the dielectriclayers 102 will act as planarization layers and offers a smoothermorphology at the interface with the metallic layer 103.

Moreover, ITO has a strong reactivity with metal layers. Even asuperficial interdiffusion for example will greatly reduce thereflective properties of the reflective structure.

The structure presented in one embodiment of the present inventionprovides a solution that reduces the direct contact between the ITO inthe reflective metal layers without significant degradation of theelectrical properties of the light-emitting device. The dielectric layeror combination of dielectric layers 102 acts as a barrier between theITO 101 and metal layers 103, and significantly improves the performanceof the reflective mirror 105 as well as the reliability of the structureover time. The dielectric layer or layers may comprise material with alower refractive index than the ITO layer, thereby improving the indexcontrast of the reflective mirror 105.

In addition, a well-tuned (to a specific optical wavelength range)combination or sequence of dielectric layers 102, combined with anadequately reflective metallic layer, provides an optimal totalreflectivity. The resulting composite mirror structure 105 provides asignificant performance improvement compared to the prior art. Itprovides a high reflectivity, low-resistance ohmic contact,composite-mirror structure.

Said dielectric layers 102 may be comprised of oxide(s), nitride(s) orfluoride(s) of any one or more of the following: Si, Nb, Ta, Al, In, Mg,Sn.

The reflective metal layer 103 can be any of the following or an alloyor other combination formed with the following: Au, Al, Ag, Ni, Cu, Pt,Pd, In.

FIG. 3 is a graphical plot of the I-V characteristics of the compositemirror of FIGS. 1 a, and 2. As shown in FIG. 3, the composite mirroralso has good I-V characteristics, so that the composite mirror does notdegrade the function of the ITO as an ohmic p side contact to the LEDactive region 100.

FIG. 4 is a graphical plot of the reflectivity of the composite mirrorof FIGS. 1 a, and 2. As shown in FIG. 4, the composite mirror of FIGS. 1a, and 2 has good reflectivity over a range of wavelengths.

Wafer Bonding Device/Process

Following the formation of the composite mirror structure, a substitutesubstrate is bonded to the growth substrate. As shown on FIG. 5, in thecase of GaN grown from sapphire wafer, the growth substrate is curveddue to the stress generated from CTE mismatch between GaN and sapphire.To generate good bonding between a flat substitute substrate and acurved growth substrate, one embodiment of the present inventionincludes:

i. A flexible and thin substitute substrate able to conform to thegrowth substrate;

ii. A wafer bonding device as shown on FIGS. 6 a and 6 b that use avacuum-sealed thin foil (Al, Cu, graphite foil, or polyimide film) toapply uniform gas pressure over the entire wafer surface.

iii. A bonding material that has good thermal and mechanical propertiesthat can perform bonding process at relatively low temperature.

The substrate transfer method of the present invention allows a widerange of choice for the new substitute substrate. The following tablelists some candidate materials for such a substitute substrate. Asmentioned above, ideally, the substitute substrate should have a CTEequal or slightly higher than the growth substrate to preserve acompressive force over LED chips. Among these carriers, AlSiC, CuW orCuMo are good candidates for this purpose. Thus, the substitutesubstrate may comprise a metal matrix composite material, where themetal matrix composite material has a CTE equal or slightly higher thanthe growth substrate materials. In one embodiment, this metal matrixcomposite material comprises one of the following: AlSiC, CuW and CuMo.

For conforming to the growth substrate during wafer bonding process, thethickness of substitute wafer is reduced for enhanced flexibility. It isworth mentioning that the final thickness of LED chips is typical in therange of 100-200 um. Therefore, thinning of the substitute substrate isnecessary before singulation of LED chips. Preferably, the substitutesubstrate is thinned to a thickness of not more than about 500 microns,or not more than about 200 microns, prior to bonding to the growthsubstrate with LEDs thereon. However, for easy handling of the bondingwafer after removing the growth substrate, a temporary support can beused.

CTE Thermal Young's (ppm/ conductivity modulus Material degree C.)(W/m-degree C.) (GPa) Si 3 150 107 Cu 17 400 131 Ni 13 91 200 Cu(20%)—Mo8 180 313 Cu(20%)—W 7 200 367 AlSiC 6-16 170-220 188

The present invention also allows a wide range of choice for the bondingmaterial. The following table lists some bonding materials. As mentionedpreviously, bonding material should have high thermal conductivity; goodmechanical property and can create good bonding at relatively lowtemperature. Among these commercial bonding materials, eutectic hardsolders such as Au/20Sn, AuSi or AuGe has good mechanical and thermalproperties, therefore can be used as bonding material for thisapplication. These hard solders, however, all contain high percentage ofgold and therefore are very expensive. In addition, these hard soldersneed fairly high processing temperature (melting range from 280 to 363C), which may cause damage to the LED properties such as thereflectivity of the composite mirror.

Coefficient of thermal Thermal line Tensile Composition Melting rangeexpansion conductivity strength System Wt % Solidus Liquidus 10⁻⁶ J/cm,S, N/mm² Au—Sn alloy Au/20Sn 280(Eutectic) 17.5 2.5 284 Au/90Sn217(Eutectic) 13.6 —  63 Au—Si alloy Au/3.15Si 363(Eutectic) 14.9 2.9255 Au/2Si 363 760 13.9 — — Au—Ge alloy Au/12Ge 356(Eutectic) 12.0 2.6186 Au/7.4Ge 356 680 13.1 — — Reference Au 1063 14.2 — 131 Pb/63Sn183(Eutectic) 24.7 0.5  48 Pb/5Sn 274 314 28.7 0.4  93

New Bonding Material

As mentioned above, commercial hard solder such as AuSn (80/20) has goodthermal and mechanical properties but are high cost and need relativelyhigh process temperature. A new cost effective bonding material isintroduced in one embodiment of the present invention. FIGS. 7 a and 7 bare binary phase diagrams of Al—Ga and Cu—Ga that illustrate the newbonding materials. The new bonding material takes advantages of the highthermal and mechanical properties of aluminum (Al) and copper (Cu) anduse gallium (Ga) to create low temperature bonding process. Heattreatment of the bonding phase above 254° C. will convert lowtemperature gallium into higher temperature intermetallic phases such asCuGa_(x) (X<=2) as well as form solid solution with aluminum (up to 10wt % of Ga can be dissolved in aluminum). This is apparent from FIGS. 7a and 7 b. As shown in FIG. 7 a, Al—Ga is either in liquid phase, or amixture of liquid and solid phases except where the amount of Ga is notmore than about 10% in weight of the solid solution of Ga with aluminum.The same is true for CuGa_(x) as shown in FIG. 7 b. Where the amount ofGa is more than about 66%, CuGa_(x) is in liquid phase, or a mixture ofliquid and solid phases. Thus, since these compounds or combinationsshould be in solid phase to be used for bonding the growth andsubstitute substrates, the amount of Ga is preferably not more than 10%in weight of the solid solution of Ga with aluminum, and not more than70% in weight in CuGa_(x).

In the present invention, the growth substrate and the substitutesubstrate are coated with Al (adhesion layer) and then with copper (usedas wetting layer as well as a layer to protect aluminum from oxidation).A thin layer of gallium metal is brushed onto the copper surface tocreate the bonding. Since the melting temperature of Gallium is ˜30° C.and the CuGa_(x) intermetallic has a melting temperature above 254° C.,bonding temperature above 254° C. can convert the low temperature Gainto high temperature intermetallic phases such as CuGa_(x) as well assolid solution into aluminum, depending on the thickness of the copperlayer and whether the Ga will reach the Al layer. If copper is not usedto cover the Al layer, than only a solid solution of Ga into aluminum isformed.

After formation of the new supporting substrate, the growth substrate 10is removed by one of the following methods or a combination of them: wetetching, mechanical grinding, or Laser Lift-Off or LLO. The laserLift-Off process is a process using selective absorption of a laserradiation to separate the epi-layers from the epitaxial substrate. Forexample, a pulsed UV-laser is shined through the epitaxial sapphiresubstrate. The wavelength of the laser radiation is chosen so that theAlInGaN epi-layers absorb the radiation but not the substrate. Thisabsorption leads to a high-temperature decomposition of theepilayers-substrate interface. In the case of AlInGaN epi-layers on asapphire substrate, the decomposition process generates plasma creatinga surpressure of N₂ gas and leaving a thin layer of Ga metal behind(melting temperature <30° C.), which allows easy separation of thesapphire substrate. As noted above, heat generated by this process maycause temperature of the LEDs to rise significantly, so that CTEmismatch may cause damage to the LEDs unless the change in stressconditions is managed successfully.

Finally, formation of the upper electrode 119 for the n ohmic contact,surface texturing 129, passivation layer 139, bonding material 106, andsubstitute substrate 107 as shown in FIG. 2 are performed to increasethe light extraction efficiency of the surface-emitting device.

FIGS. 8 a and 8 b show the SEM photos of LED chips mounted on AlSiCsubstitute substrate before and after the singulation process.

The process of making the LEDs is illustrated in the flow chart of FIG.9, in reference to the device or apparatus of FIGS. 6 a and 6 b. Anepitaxial wafer is first provided, where the wafer comprises a growthsubstrate such as a sapphire or GaAs substrate, with one or more LEDstructures grown on it. (Block 202, FIG. 9) This epitaxial wafer maycomprise only the LED epitaxial layers on the growth substrate and thusonly the layers 10 and 100 in FIG. 1 a. A composite mirror 105 is coatedonto the p-side of the epitaxial wafer, forming the structure shown inFIG. 1 a. (Block 204, FIG. 9) The substitute wafer is thinned asdescribed above using a grinder to the desired thickness. (Block 206,FIG. 9) The bonding surface of the substitute wafer and the p-side ofthe epitaxial wafer are coated with a bonding material, such as a layerof Al, followed by a layer of Cu and then brushed with Ga, which may bein liquid or semi-liquid form at room temperature. (Block 208, FIG. 9)The coated surfaces of the substitute wafer and of the epitaxial waferare then placed in contact to form a wafer assembly 252 (FIG. 6 b) andplaced in a bonding and pressure chamber 254 in FIG. 6 b. FIG. 6 a is anexploded view of the actual bonding device; as shown in the diagram, thegas pressure is maintained by tightening multiple screws 253 against anO-ring (not shown in FIG. 6 a) in groove 255 around the wafer assembly252. The vacuum chamber (conduit 256 a and the space between the heatspreader 256 and the substitute wafer, described below) and pressurechamber 254 are separated by a thin foil such as polyimide, Al or Cufoil 258 shown in FIG. 6 b (not shown in FIG. 6 a).

The wafer assembly 252 is placed on top of a copper heat spreader 256,and covered by a thin foil made of polyimide, Al or Cu 258 which servesas a boundary between the vacuum chamber and pressure chamber 254. Gasis supplied to pressure chamber 254 through inlet 260 and exits throughoutlet 262, at a pressure preferably from 15 to 200 psi. The pressure isthen exerted isotropically onto the epitaxial wafer portion of the waferassembly 252 through foil 258 towards the thinned substitute wafer. Inan alternative embodiment, pressure may be exerted on the substitutewafer towards the epitaxial wafer instead by turning the wafer assembly252 upside down; all such and other variations are within the scope ofthe invention.

Heater block 256 (not drawn to scale in FIG. 6 b) has a conduit 256 atherein through which gas in a region on a side of the substitute waferin the wafer assembly 252 may be pumped out by means of a vacuum pump(not shown) to reduce the pressure in the conduit 256 a and the spacebetween the heat spreader 256 and the substitute wafer. This furtheraccentuates the pressure differential between the chamber 254 and thevacuum chamber underneath the wafer assembly 252, thereby causing thesubstitute wafer in assembly 252 to flex and conform in shape to thecurved shape of the epitaxial wafer. A heater 270 heats the bondingmaterial in the wafer assembly 252 to a desired temperature, causing thecombination of Ga with Al and/or Cu to form as described above. (Block210, FIG. 9) The growth substrate is removed as described above. (Block212, FIG. 9) The n-side contacts and surface textures are formed,followed by mesa etch on epi layer to ensure P side and N side of theepi are electrically isolated. Finally, a passivation layer is coatedover the PN junction to prevent shorting by contaminations. (Block 214,FIG. 9) The resulting wafer is then singulated. (Block 216, FIG. 9)

While the invention has been described above by reference to variousembodiments, it will be understood that changes and modifications may bemade without departing from the scope of the invention, which is to bedefined only by the appended claims and their equivalents.

1. A solid-state light emitting structure comprising: a LED comprising aplurality of semiconductor epitaxial layers, said semiconductorepitaxial layers having been grown on a growth substrate; a substitutesubstrate different from the growth substrate supporting said LED; and abonding material bonding said substitute substrate to said LED, saidbonding material not including gold or tin, wherein said bondingmaterial includes Ga, and a combination of Ga and Al or Cu.
 2. Thesolid-state light emitting structure of claim 1, said bonding materialcomprising AlGa, wherein amount of Ga in said AlGa is not more thanabout 10%.
 3. The solid-state light emitting structure of claim 1, saidbonding material comprising CuGa_(x), wherein amount of Ga in saidCuGa_(x) is not more than about 70%, x is equal or less than
 2. 4. Asolid-state light emitting structure comprising: a LED comprising aplurality of semiconductor epitaxial layers, said semiconductorepitaxial layers having been grown on a growth substrate; a substitutesubstrate different from the growth substrate supporting said LED; and abonding material bonding said substitute substrate to said LED, saidbonding material not including gold or tin, wherein said bondingmaterial includes a solid solution of Ga and Al and an intermetallicphase of Ga and Cu.
 5. A solid-state light emitting structurecomprising: a LED comprising a plurality of semiconductor epitaxiallayers, said semiconductor epitaxial layers having been grown on agrowth substrate; a substitute substrate different from the growthsubstrate supporting said LED; and a bonding material bonding saidsubstitute substrate to said LED, said bonding material not includinggold or tin a current spreading layer between the substitute substrateand the LED, said current spreading layer in contact with said LEDserving as ohmic contact with the LED; and a buffer layer between thecurrent spreading layer and the substitute substrate, said buffer layercomprising a plurality of vias, said buffer layer having a refractiveindex that is below that of the current spreading layer.
 6. Thesolid-state light emitting structure of claim 5, further comprising areflective metal layer between the buffer layer and the substitutesubstrate, said reflective metal connected to the current spreadinglayer through said vias in the buffer layer.
 7. The structure of claim6, said reflective metal layer is any of the following or combination ofthe following: Au, Al, Ag, Ni, Cu, Pt, and Pd.
 8. The structure of claim5, said current spreading layer is any of the following or combinationof the following: ITO, Ni/Au, and RuO₂.
 9. The structure of claim 5,said buffer layer comprising a dielectric material.
 10. A solid-statelight emitting structure comprising: a LED comprising a plurality ofsemiconductor epitaxial layers, said semiconductor epitaxial layershaving been grown on a growth substrate; a substitute substratedifferent from the growth substrate supporting said LED; and a bondingmaterial bonding said substitute substrate to said LED, said bondingmaterial not including gold or tin, said substitute substrate comprisinga material with thermal conductivity greater than about 100 W/m degreesCentigrade.
 11. The structure of claim 10, said substitute substratecomprising a metal matrix composite material, said metal matrixcomposite material having a coefficient of thermal expansion equal orslightly higher than the growth substrate materials.
 12. The structureof claim 11, said metal matrix composite material comprising one of thefollowing: AlSiC, CuW and CuMo.